The present invention relates to a method for mounting a chip, and specifically to a chip mounting method for mounting a chip directly onto a substrate in a form of face down and performing underfill between the chip and the substrate.
A chip mounting method is well known, wherein a bump is formed on a chip by a solder and the like, the chip is approached to a substrate in a form of face down, and after the bump is brought into contact with an electrode of the substrate, the bump of the chip is heated and molten to be bonded to the electrode of the substrate. An underfill agent, which is composed of a nonconductive adhesive, is injected into a gap between the chip and the substrate in order to increase the total bonding strength between the chip and the substrate. Further, the reliability of the electrical insulation between the bonded portions is increased by a condition where the underfill agent is filled around the bump of the chip and the electrode of the substrate bonded to each other.
In a conventional chip mounting method for forming a solder bump, firstly a flux is applied to the bump or a substrate side, and after the chip is mounted on the substrate at a predetermined position, the solder bump is molten by heating (in most cases, associated with pressing) and the bump is preliminarily bonded to the electrode of the substrate. After being cooled, the flux is removed by cleaning, and thereafter, an underfill agent is injected into a minute gap between the chip and the substrate from the side direction. After the underfill agent is injected, heating is performed again, the underfill agent is re-flown and thereafter cured.
In such a conventional chip mounting method, however, when the solder bump is heated and molten in the preliminary bonding process of the chip and the substrate, there is a fear that the bonded portion is secondarily oxidized by the surrounding atmosphere. If such a secondary oxidation occurs, it may decrease the reliability of the electrical bonding between the bump and the electrode of the substrate. In order to prevent the secondary oxidation, there is a method for purging the atmosphere around the bump using nitrogen gas and the like when the bump is bonded with the electrode. However, such purging by nitrogen gas and the like causes a chip mounting apparatus to become large-sized, the mounting apparatus and the mounting process to become complicated, and the cost thereof to increase, and because it is necessary to take a time for the purging by nitrogen gas and the like, a high-speed mounting may be obstructed.
Further, in the above-described conventional mounting method, because it is necessary to apply a flux and remove the flux by cleaning after the bonding for melt-bonding of the solder bump, there is a problem that the number of processes until completion of main bonding is fairly large. Moreover, the perfect cleaning of the flux is difficult, and therefore, the residual components may reduce the reliability of the bonding. In order to achieve a fluxless bonding, proposed is a method for using an alcoholic organic material having a melting point higher than that of a solder instead of a conventional flux, and removing it by vaporizing without cleaning it after the preliminary bonding (JP-A-8-293665). However, in this method, basically only the cleaning process of the flux becomes unnecessary, and the method is poor in effect for reducing the number of the processes in the bonding. In particular, the processes for injecting an underfill agent after the preliminary bonding, for re-flowing the underfill agent and for curing the underfill agent still remain as they are. Further, in this method, as the bump pitch has been in a fine pitch condition, it has become difficult to inject the underfill agent.
Accordingly, an object of the present invention is to provide a chip mounting method which can prevent the secondary oxidation of a bump to be heated and molten without performing the purging by nitrogen gas and the like, and which enables to reach a main bonding process at a time with a fluxless condition, thereby simplifying both the mounting apparatus and process greatly.
To accomplish the above object, a method according to the present invention for mounting a chip by bonding a bump formed on at least one of the chip and a substrate to an electrode formed on the other and providing an underfill agent between the chip and the substrate, comprises the steps of applying the underfill agent onto at least one of the substrate and the chip, moving the chip to the substrate to bring the bump into contact with the electrode and to expand the underfill agent, in a space between the chip and the substrate, to around the bump and the electrode in contact with each other, and heating the bump or electrode in a state that the bump is buried in the underfill agent to melt the bump or electrode so as to weld the bump to the electrode. Although the material of the bump or the electrode is not particularly restricted, the present invention is suitable for mounting especially in a case where it is formed from a solder.
In the present invention, the bump is formed on at least one of the chip and the substrate. Namely, there are (A) a case where the bump is formed on the chip and the electrode is formed on the substrate, (B) a case where the bump is formed on the substrate and the electrode is formed on the chip, and (C) a case where the bump is formed on each of the chip and the substrate. Therefore, in the case of (C), the xe2x80x9celectrodexe2x80x9d according to the present invention means a technical concept including a bump.
In the chip mounting method according to the present invention, it is preferred that the underfill agent is expanded up to a side portion of the chip by further pushing the chip toward the substrate together with heating the bump or electrode or after the heating. Particularly, it is preferred that a fillet is formed at the side portion of the chip by the underfill agent expanded up to the side portion of the chip. By such an underfill agent expanded sufficiently broad, the bonding strength between the chip and the substrate can be increased as well as a desired excellent electrical insulation can be ensured.
Further, it is preferred that, before the chip is mounted to the substrate, at least one of the bump and electrode is treated for preventing a primary oxidation thereof, that may be generated on at least one of the bump and electrode by the time of the mounting by the atmosphere during storage and the like, or that may be generated associated with a certain pre-treatment such as heating treatment for forming a solder bump and the like. As the primary oxidation preventing treatment, for example, a treatment can be employed, wherein the bump or the electrode is cleaned by energy wave or energy particles. As the energy wave or energy particles, for example, any of plasma, ion beam, atomic beam, radical beam and laser can be used. Further, as the primary oxidation preventing treatment, for example, treatments can also be employed, wherein a substituent for removing oxygen such as a fluoro group is chemically bonded to the surface of the bump or electrode, thereby forming a anti-oxidation surface layer, wherein an oxide is removed from the surface by reducing operation due to hydrogen, and wherein gold is plated to the surface.
In the above-described chip mounting method according to the present invention, the underfill agent is applied before chip mounting, and before heating, the applied underfill agent is expanded so as to bury therein the bump and the electrode of the substrate in contact with each other, and therefore, when the bump is heated and molten, the bump is cut off from the surrounding atmosphere by the underfill agent, and the secondary oxidation of the bump can be prevented. Therefore, the purging due to nitrogen gas and the like for preventing the secondary oxidation such as a conventional method becomes unnecessary, and the apparatus and the process can be greatly simplified.
Further, if the aforementioned primary oxidation preventing treatment is carried out, the application of flux becomes unnecessary, and a further simplification becomes possible. Because the cleaning of flux after bump bonding also becomes unnecessary by making the application of the flux unnecessary, the process can be simplified also from this point of view.
This chip mounting method according to the present invention is not a method for injecting an underfill agent into a gap between a chip and a substrate after preliminarily bonding a bump and an electrode and thereafter mainly bonding them by heating as in a conventional method, but a method for expanding an underfill agent to around a bump of chip and an electrode of a substrate in contact with each other, and performing a main bonding at a time without performing a preliminary bonding by heating the bump or the electrode in the expanded underfill agent. Because this bonding is carried out through heating and melting of the bump or the electrode in the underfill agent which does not come into contact with oxygen, as mentioned above, there is no fear that the secondary oxidation occurs by heating. Besides, during this heating and melting, re-flow necessary for the bump or the electrode is carried out at the same time. Therefore, in the method according to the present invention, the mounting is completed substantially by two processes of the primary oxidation preventing treatment process and the heating and bonding process of the chip and the substrate including the application of the underfill agent, thereby greatly reducing the number of the processes for chip mounting and greatly simplifying both the process and the apparatus.
Furthermore, if the underfill agent is expanded up to the side portion of the chip by further pushing the chip toward the substrate after the bump or the electrode is softened or molten by heating, the bonding strength and the retaining strength of the chip relative to the substrate can be increased. Especially, if the underfill agent is expanded so as that a fillet can be formed on the side portion of the chip, by forming the fillet, invasion of moisture and the like into a portion between the chip and the substrate can be prevented by the underfill agent forming the fillet in addition to the underfill agent present between the chip and the substrate, and at the same time, it becomes possible to increase the retaining strength and the bonding strength of the chip. Further, because a fear that the underfill agent remains between the bump and the electrode can be removed by the above-described pushing, a more stable electric connection can be achieved therebetween.
Thus, in the chip mounting method according to the present invention, a secondary oxidation of the bump bonding portion can be prevented without using nitrogen gas and the like as in a conventional method, both the apparatus and the process can be greatly simplified, and a chip mounting having a high reliability in bonding and insulation properties can be carried out inexpensively.